why my curve shape become smaller than original source curve?
Curve control point's Radius ≠  1

Each control point has a parameter called Radius that controls the size of the object being beveled in the area of influence of that control point. You can see from the image sequence below how the parameter can be varied to alter the size of the section without a taper object.



In your specific case, the Mean Radius of the curve that can be seen in the Transform section of the Property Shelf is saying that the average Radius is different than 1 (size untouched).



By examining each control point you can see that none of them is set to 1, that's why the size of the beveling object is different from what expected.

The solution consist in restoring the Radius to 1 by calling the Set Curve Radius operator while all the control points selected in edit mode. 

Result: